The present invention relates in general to electromagnetic energy collection and processing systems, and is particularly directed to a method and apparatus for generating a two-dimensional image of a terrestrial region of interest, by passively collecting and processing scattered radio waves illuminating the terrestrial region from a plurality of narrowband RF emission sources, such as commercial television transmission towers, that effectively correspond to a composite wideband illumination source (e.g., of the type employed in synthetic aperture radar applications).
Conventional schemes for generating images of objects or scenes include a variety of energy illuminating and collection methodologies, such as visible and infrared light-based processes (e.g., photography), and coherent electromagnetic radiation-based processes (e.g., synthetic aperture radar (SAR) and holography). While conventional (non-coherent) light-based photography allows image capture of exterior surfaces of objects in a scene, it does not create an image of where the light cannot go (behind the exterior surface of an object, such as into the interior of a building or beneath a vegetation canopy, in the case of visible light).
Synthetic aperture radar and holography use coherent electromagnetic radiation (e.g., narrow bandwidth radar pulses in the case of SAR and coherent light in the case of holography) to construct an image. Advantageously, because it processes volume-based (rather than planar-based) differential phase information, holography is able to provide for the generation of a three-dimensional image of an object. Still, its use to date has been essentially limited to controlled, volume-constrained static environments, such as an opto-physics laboratory.
There are many terrestrial regions, such as cities, industrial areas, and the like, containing a wide variety of cultural features, such as buildings, bridges, towers, etc., as well as interior components thereof, for which images (including those captured at different times for determining the presence of environmental changes) are desired by a variety of information analysis enterprises. Curiously, many if not most of such terrestrial regions are continuously illuminated by one or more relatively powerful narrowband radio frequency (RF) transmitters, such as television broadcast towers, creating a condition known as xe2x80x98RF daylightxe2x80x99. Because of the partial transparency to such RF emissions (especially at and below VHF and UHF frequencies) of many objects, including both natural vegetation and man-made structures, these RF-daylight signals can be expected to be reflected/scattered off cultural features (including both exterior and interior surfaces) of an illuminated region.
Advantageously, the invention described in the above-referenced ""637 application takes advantage of this RF daylight phenomenon, by passively generating, for multiple points within a prescribed three-dimensional volume illuminated by a coherent RF transmitter, such as a commercial television transmission tower, RF reflectance/scattering coefficient values from which a three-dimensional image of cultural features within the illuminate volume may be derived.
For this purpose, the invention described in the ""637 application contains a front end, RF energy collection section having a reference signal collector (antenna) which collects non-scattered RF energy emitted by an RF reference source illuminating the potentially cultural feature-containing terrestrial region of interest. A second, dynamic scattered image energy collector, mounted on a platform overflying the illuminated terrestrial region, collects RF energy that has been scatteredxe2x80x94reflected from various points of cultural features (such as buildings and contents thereof) within a three-dimensional volume of space containing the terrestrial region.
Dynamic collection of the scattered RF image energy is conducted over plural non-coincident travel paths (such as those extending from horizon-to-horizon), to ensure that energy collected from the illuminated region will be derived by way of multiple three-dimensionally offset views, and provide the resulting aperiodic lattice additional power to resolve image ambiguities and enhance the three-dimensional imaging capability. Once captured by their respective energy receiver sections, the RF reference signal energy and the RF image energy are digitized and stored, so that they may be readily coupled to a correlation-based image data processing section.
The correlation based image data processing section assumes that the source of RF energy illuminating the three-dimensional spatial volume of interest is located at some fixed location in space. Where the scattered RF energy collector is used to simultaneously collect non-scattered energy emitted from the reference signal source, termed a xe2x80x98self-referentialxe2x80x99 embodiment, the received signal y(t) produced by the RF energy collector contains the direct path signal from the illumination source to the collector plus time-delayed, Lorentz-transformed RF energy scattered from the illuminated region.
Because the coordinates of the source of the reference signal are spatially displaced from the location of a respective illuminated point, there is a time delay associated with the reference signal""s travel path from the illumination source to the potential scattering location, and also a time delay associated with the reference signal""s travel time from the reference signal source to the RF energy collection aperture. In addition, there is a time delay associated with the travel time of the RF energy scattered from the illuminated location to the scattered image energy collector. In order correlate the reference signal with the RF energy signal received by the moving collector, it is necessary to account for these delays, as well as the time-scaling of the signal received by the energy collector resulting from the fact its platform is moving relative to the illuminated location.
The signal received at the dynamic collector is subjected to a first Lorentz transform and delay operation to transform the reference signal component of the energy received at the collection aperture to the illuminated location. The received signal is further subjected to a second Lorentz transform which accounts for signal propagation delay and performs a Lorentz transform from its moving frame of reference to the static frame of reference of the illuminated point. In the self referential embodiment, the received signal at the dynamic collection aperture contains the reference illumination signal, which is removed/nulled out by means of a reference signal suppression operator.
The reference signal is then correlated with the scattered signal over a relatively long integration interval, such as one on the order of several tens of seconds to several tens of minutes, or longer, sufficient to collect enough valid scattering energy associated with a prescribed signal-to-noise ratio, with scattered energy values associated with RF frequency from the reference source illuminating the scattered location constructively combining, whereas all others destructively cancel. This produces, for the illuminated location, a scattering coefficient which is a complex interference pattern (containing both amplitude and phase components) containing all the information necessary to recreate a three-dimensional monochromatic image of the illuminated scene.
The output of the correlator may be coupled to a downstream image utility subsystem for generation of the three-dimensional image of the scene. The resolution to which the illuminated scene may be imaged (three-dimensionally) is limited by the Rayleigh wavelength (i.e., one-half the wavelength) of the illuminating reference source.
Now, even through the image generation scheme of the ""637 application described above provides the ability to passively collect and process RF energy emitted by a relatively powerful RF illumination source into a three-dimensional image of cultural features of a dynamically observed terrestrial region of interest, it can be expected to require a relatively lengthy period of time (e.g., on the order of several to tens of minutes of more) for collection (typically over multiple passes along two or more mutually three dimensionally offset observation paths extending horizon-to-horizon) of sufficient data that can be correlated for the generation of an image whose image points have an acceptable signal-to-noise ratio.
In accordance with the present invention, this relatively long energy collection period can be substantially reduced by using an energy illumination and collection aperture that effectively corresponds to that employed in synthetic aperture radar (SAR) applications. Rather than collecting energy emitted from a single narrowband illuminating RF source from multiple views associated with relatively lengthy (e.g. horizon-to-horizon) energy collection paths, the present invention collects RF energy scattered by cultural features with a spatial volume illuminated by a plurality of spectrally different narrowband RF emission sources, having a spectrally composite waveform that is functionally equivalent to a wideband illumination source.
The use of such a composite wideband RF signal source enables a first dimension of cultural features of the illuminated region to be resolved to a relatively fine resolutionxe2x80x94on the order of that obtainable for range measurements in SAR systems. A second dimension of the imaged region corresponds is generally orthogonal to the range dimension, and corresponds to the azimuth component of the collected wideband energy signal. As in an SAR system, the extent to which the azimuth component is resolvable is defined by the spatial energy collection window along a single travel path of the scattered energy collector""s dynamic platform, as it overflies the illuminated region. Such a wideband energy collection window is typically on the order of a few to several tens of seconds, rather than minutes. As a result, the invention is able to produce scattering coefficients associated with the viewed scene (in two dimensions) in a relatively short period of time.
Not only may the respective RF frequencies of the various illumination sources not necessarily spectrally overlap or be spectrally contiguous, but they can be expected to mutually non-coherent. This means that, in order to realize a useful image, the phase components of the scattering coefficients obtained from the viewed region of interest for each illumination frequency must be adjusted to correct for their mutually differential offsets. To correct for this phase incoherence, a respective set of scattering coefficient data obtained for each illumination source is applied to a cultural feature extraction operator, such as a conventional edge detection operator, in order to locate a relatively strong cultural feature that is spatially common to multiple images.
The extracted cultural feature is used as a commonality connector to provide a phase coherence correction base for plural sets of scattering coefficient data. In particular, the extracted cultural feature is applied to a standard electromagnetic waveform analysis tool, that is operative, for each narrowband RF signal source for which the cultural feature has been identified, to calculate a set of scattering coefficients that should be theoretically produced as a result of an illumination by that frequency of spatial points that lie along the extracted cultural feature. The phase values of these calculated scattering coefficients are then compared with the actual measured phase values of the scattering coefficients as determined for RF energy passively collected from the dynamic collection aperture for the illumination source at the corresponding frequency.
The difference between the phase values of the two sets of coefficients (tool-calculated and scattered energy collection-based) for the extracted cultural feature represents a phase offset value that needs to be imparted to the measured scattering coefficient values for all spatial points in the illuminated region. Once this phase correction has been made for a given illumination frequency, all of the scattering coefficients for the set of data associated with that particular illumination frequency are effectively tied to a common phase coherence reference. This allows the scattering coefficients of that narrowband frequency set to be coherently combined with those of another spectrally different narrowband set of scattering coefficients whose phase components have been similarly corrected, based upon the same extracted cultural feature. Namely, the extraction of a cultural feature that is common to two or more sets of scattering coefficient data respectively associated with two or more illumination frequencies is used as a spatial reference for enabling phase coherence adjustment of all of the data points of each set for those illumination frequencies.
It should be noted that a given cultural feature that is common to two or more data sets may not be common to the data sets for all frequencies. In this case, one or more additional cultural features may need to be extracted in order to phase-coherence link all of the data sets together. Once each of the measured sets of scattering coefficients have been phase corrected, as described above, they may be coherently combined to provide a composite scattering coefficient data set, from which a two-dimensional image of the viewed scene (in terms of azimuth and range dimensions) may be generated.